The present invention relates to methods for introducing crack arrest behavior into a brittle material by designing particular types of residual stress profiles in the material.
Brittle materials, such as ceramics and inorganic glasses, are sensitive to surface contact damage, which gives rise to flaws that reduce strength and leads to a large variation in strength from specimen to specimen. These materials usually fail in an unstable and catastrophic manner, with no advanced warning, when subjected to sufficiently high mechanical and/or thermal stresses. When such materials are tested in bending, uniaxial tension, or other types of tensile stress fields, a single flaw typically forms into a propagating crack that grows rapidly and in an unstable manner. Extensive damage may also occur in a thermal shock (rapid change in temperature) type of loading and lead to multiple cracks. When failure occurs, there is no forewarning and the material may splinter and cause harm. It would therefore be useful to be able to find ways to reliably arrest cracks in such brittle materials.
One approach for improving the mechanical reliability of brittle materials is to increase their strength. Surface compression is known in the art to improve the surface contact damage resistance and strength of many brittle materials, due to the increased energy required to propagate a crack through the region of compression. Silicate glasses may be thermally tempered or chemically strengthened as known in the art to introduce such compressive stresses into the surface.
A typical thermal tempering process may include heating the object to a temperature in which internal stresses can be relaxed without deformation. The object is then quickly cooled. The significantly different cooling rates for the surface versus the center regions of the object lead to a residual stress at the surface.
Typical chemical strengthening processes may introduce surface compression by two methods. In one method, the surface region is chemically modified such that it has a lower thermal expansion coefficient than the underlying material so that during cooling, it is placed in residual stress. In another method, a chemical reaction can be used to increase the volume of the surface region by processes such as ion exchange, phase transformations and oxidation. Provided that this process can be carried out without any major stress relaxation, the increased volume of the surface region of the object will give rise to residual compression. Another method for introducing compressive stresses into an object is to apply a coating having a lower coefficient of thermal expansion than the material being coated. When applied at an elevated temperature, upon cooling, the surface will be placed into residual compression. One benefit of forming surface compression is that it often leads to a minimum strength value. However, a limitation is that failure of the material will still be catastrophic under sufficient loading conditions.
In addition, introduction of a surface compression may lead to an increase in the strength variability. In one study on the ion exchange strengthening of silicate glass, the coefficient of variation in the strength increased by almost a factor of 2 for a sixfold strength increase. Increased strength variability leads to difficulties in the design process, and is an obstacle to the engineering use of these materials. For brittle materials, design engineers often need to assure the mechanical reliability in terms of very small failure probabilities at a prescribed design stress level for a given lifetime. It would be desirable to identify processes that can be used to increase strength and decrease strength variability.
One preferred embodiment relates to a method including determining a largest flaw size in a material and introducing compressive residual stress into said material in a manner so that the absolute value of said compressive stress increases from a first value to a second value within a depth in the material. The first value of residual stress is controlled to be less compressive than the second value, and the depth of the residual stress is greater than the largest flaw size. In addition, the first value of residual stress is selected to provide a stress gradient that results in multiple cracking of the material without failure of the material when subjected to a sufficient applied stress.
Another embodiment relates to a method for forming a glass which displays visible cracking prior to failure when subjected to an applied stress level that is greater than a predetermined minimum stress level and less than an ultimate stress level for failure. The method includes determining a critical flaw size in the glass and introducing a residual stress profile to the glass so that a plurality of visible cracks are formed prior to failure in the glass when the glass is subjected to a stress that is greater than the minimum stress level and lower than the ultimate stress level, by calculating an apparent fracture toughness curve from the residual stress profile and selecting the applied stress level to be in a region of inflection along the apparent fracture toughness curve.
Still another embodiment relates to a method for introducing residual stress into a material including performing a first ion exchange to a first depth to exchange a plurality of first ions with a plurality of second ions having a larger volume than the first ions.
The method also includes performing a second ion exchange to a second depth to exchange a plurality of second ions with a plurality of third ions having a smaller volume than the second ions. The first depth is controlled to be greater than the second depth.